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SN65LBC180: DE and RE pin settings for RS485 Half duplex operation

Aagneya C
Prodigy 20 points
Part Number: SN65LBC180


Tool/software:

We are using the IC SN65LBC180 in  our design for RS485 communication and generally full duplex IC can be used as an half duplex IC by shorting the positive and the negative signals together.

during our testing of Half duplex we had the below observations.

->for transmission to occur the RE pin and DE pin both where set to low.

->for reception to occur the RE pin was set to low and the DE is set to high.

 

Kindly confirm if the setting of the DE and the RE for the transmission and reception is expected? 

  • 0
    Guru 317480 points

    For transmission to occur, DE must be high.
    For reception to occur, /RE must be low.

    It is possible to transmit and receive at the same time, if you want to check that what you're sending is visible on the bus.

  • 0
    TI__Guru 74961 points

    Hi,

    Clemens is correct.

    If you want to transmit, DE should be held high to enable the drivers. 

    For receiving, you want #RE to be low. 

    It's possible to set DE high and #RE low at the same time though you would end up seeing what you send on the Routput. If your software knows this, it can throw out that data which would save you from toggling the DE/#RE pins.

    -Bobby

  • 0 in reply to BOBBY
    Prodigy 20 points

    Yes sorry! i had jumbled them up, the DE pin is set to high for transmission and the /RE pin is set to low. During reception the RE and DE both are set to low.

    wanted to know what should be the state of /RE pin during transmission, is it okay to set it as high or should we set it as low.

    and transmit with recvieve is only for the checking the what the IC has sent correct? or does it mean that we can do transmit to a slave and recieve data from the slave simultanouesly (half duplex does not support this correct?)

    Regards,

    Aagneya C

  • 0 in reply to Aagneya C
    Guru 317480 points

    It is not possible for two nodes to send at the same time. You would receive the data you're transmitting only to check that there are no conflicts or other errors.

  • 0 in reply to Aagneya C
    TI__Guru 74961 points
    Aagneya C said:
    wanted to know what should be the state of /RE pin during transmission, is it okay to set it as high or should we set it as low.

    It is okay to have RE# enabled during transmission but it doesn't really help you much. In theory, you know what data you are sending so with RE# enabled then you would see it reappear in the Routput. The MCU/processor would need to throw the data that appears on it out. The only thing I can see this being useful for is if you transmit data and see the data appearing on the Routput to be different. This may tell you there is something going on with the RS485 bus like a short or something else is transmitting when you are. 

    Aagneya C said:
    does it mean that we can do transmit to a slave and recieve data from the slave simultanouesly (half duplex does not support this correct?)

    Correct, Clemens highlighted this in his comment but in RS485 half duplex you shouldn't transmit from two nodes at the same time. It would likely result in contention. RS485 drivers can handle this situation (they are spec'd to be able to survive it) but the data integrity would be lost.

    -Bobby

  • 0 in reply to BOBBY
    Prodigy 20 points

    Thank you for the information! really helpful!